How to build a £600 Gaming PC at Overclock3D

We've often talked amongst ourselves about what the difference is between a super high-end system and one that's much more affordable, especially for gaming. One of the major things we've spoken of when reviewing graphics cards is that the speed and capabilities of the processor doesn't make that much difference when compared to the abilities of the card.
Obviously if you're going to spend your life editing video or doing an independent version of a Pixar film then you'll need all the power that you can get your hands on. But what if you just want to have a standard PC and use it for standard things, with the occasional game thrown in?